Shlomo Sasson

Adenosine Monophosphate-Activated Protein Kinase (AMPK) as a New Target for Antidiabetic Drugs: A Review on Metabolic, Pharmacological and Chemical Considerations

In view of the epidemic nature of type 2 diabetes and the substantial rate of failure of current oral antidiabetic drugs the quest for new therapeutics is intensive. The adenosine monophosphate-activated protein kinase (AMPK) is an important regulatory protein for cellular energy balance and is considered a master switch of glucose and lipid metabolism in various organs, especially in skeletal muscle and liver. In skeletal muscles, AMPK stimulates glucose transport and fatty acid oxidation. In the liver, it augments fatty acid oxidation and decreases glucose output, cholesterol and triglyceride synthesis. These metabolic effects induced by AMPK are associated with lowering blood glucose levels in hyperglycemic individuals. Two classes of oral antihyperglycemic drugs (biguanidines and thiazolidinediones) have been shown to exert some of their therapeutic effects by directly or indirectly activating AMPK. However, side effects and an acquired resistance to these drugs emphasize the need for the development of novel and efficacious AMPK activators. We have recently discovered a new class of hydrophobic D-xylose derivatives that activates AMPK in skeletal muscles in a non insulin-dependent manner. One of these derivatives (2,4;3,5-dibenzylidene-D-xylose-diethyl-dithioacetal) stimulates the rate of hexose transport in skeletal muscle cells by increasing the abundance of glucose transporter-4 (GLUT-4) in the plasma membrane through activation of AMPK. This compound reduces blood glucose levels in diabetic mice and therefore offers a novel strategy of therapeutic intervention strategy in type 2 diabetes. The present review describes various classes of chemically-related compounds that activate AMPK by direct or indirect interactions and discusses their potential for candidate antihyperglycemic drug development.

Regulation by metformin of the hexose transport system in vascular endothelial and smooth muscle cells

1 The effect of the biguanide metformin on hexose transport activity was studied in bovine cultured aortic endothelial (BEC) and smooth muscle cells (BSMC). 2 Metformin elevated the rate of hexose transport determined with 2‐deoxyglucose (2DG) in a dose‐ and time‐dependent manner in both cell types. Similar ED50 values (0.8‐1.0 mM) were determined for the effect of metformin on 2DG uptake in both BEC and BSMC following 24 h exposure to increasing concentrations of metformin, with maximal stimulation at 2 mM. 3 In BEC, metformin increased the hexose transport rate 2–3 fold at all glucose concentrations tested (3.3–22.2 mM). In BSMC incubated with 22.2 mM glucose, metformin elevated the hexose transport ∼2 fold. The drug was also effective at lower glucose levels, but did not exceed the maximal transport rate observed in glucose‐deprived cells. 4 Similar results were obtained when the effect of metformin on hexose transport activity was assessed with the non‐metabolizable hexose analogue, 3‐O‐methylglucose, suggesting that the drug affects primarily the rate of hexose transport rather than its subsequent phosphorylation. 5 The metformin‐induced increase in hexose transport in BSMC treated for 24 h with the drug correlated with increased abundance of GLUT1 protein in the plasma membrane, as determined by Western blot analysis. 6 These data indicate that in addition to its known effects on hexose metabolism in insulin responsive tissues, metformin also affects the hexose transport system in vascular cells. This may contribute to its blood glucose lowering capacity in patients with Type 2, non‐insulin‐dependent diabetes mellitus.

Autoregulation of glucose transport: effects of glucose on glucose transporter expression and cellular location in muscle.

Decreased peripheral utilization of glucose is an important pathogenic mechanism in diabetes. Although it is universally acknowledged that insulin resistance plays a major role in the reduction of glucose consumption by peripheral tissues, and many defects have been described in the function of insulin receptors (see chapter by Olefsky), indirect clinical and in vivo experimental observations suggest that hyperglycemia per se may participate in inducing and/or maintaining a reduced glucose uptake (summarized in chapter by De Fronzo). The idea occurred to us some years ago that a certain analogy may exist between the downregulation of hormone receptors by augmented hormone concentrations, and the reduction of glucose uptake by hyperglycemia. The extraordinary redundancy of compensatory events that operate in vivo make the testing of such a hypothesis near-impossible. We therefore chose to work in vitro, and because muscle is the main glucose consumer of the periphery, we focused on in vitro muscle preparations and myocyte lines.

Failure of fenfluramine to affect basal and insulin-stimulated hexose transport in rat skeletal muscle

Fenfluramine is an effective appetite suppressant that mediates its action via serotoninergic neurons. We studied the effect of pure d- and l-fenfluramine on in vitro hexose transport in isolated rat soleus muscles and skeletal muscle cells in culture. We found no evidence to suggest that the fenfluramine enantiomers affect the basal transport activity. Furthermore, the drugs did not interfere with the ability of glucose to regulate its own transport. Muscle responsiveness to insulin was not altered by the enantiomers, nor did insulin unmask any effect of fenfluramine on muscle hexose transport. These conclusions are based on experiments performed with a wide concentration range of drug and insulin, from the therapeutic to suprapharmacological levels. We discuss our results in view of published data on the effects of fenfluramine on peripheral glucose metabolism.

Erratum to: Adenosine Monophosphate-Activated Protein Kinase (AMPK) as a New Target for Antidiabetic Drugs: A Review on Metabolic, Pharmacological and Chemical Considerations by Gruzman et al., Rev Diabet Stud 2009, 6(1):13-36

In the legends to Figures 2 and 3 of the article by Gruzman et al. in the spring issue 2009 of The Review of Diabetic Studies, pages 13-36, the copyright permission for the figures refer to reference [132], and not [135] as published. The references are published below. The correct text for the legend of Figure 2 on page 21 is: n nFigure 2. D-Xylose and Compounds 19, 21 and 24 activate AMPK. A: L6 rat myotube cultures were washed and received fresh medium supplemented with 2% (v/v) FCS, 23.0 mM D-glucose supplemented with 20 mM of D-xylose (D-xyl), 5 μM of Compound 19, 150 μM of Compound 21 or 50 μM of Compound 24. These compounds were present in the medium for 40 min, 12 h, 30 min and 2 h, respectively. Control myotubes received the vehicle (V) only. AICAR (4 mM), 100 nM of insulin (Ins) and 0.25 M of D-sorbitol (S) were present for 1h, 20 min and 30 min, respectively. Whole cell lysates were prepared and Western blot analyses were performed with antibodies against AMPKα and pThr172-AMPKα. B: Human myotubes were treated as described above and taken for Western blot analysis of AMPKα and pThr172-AMPKα. Representative blot and a summary of n = 3 (* p < 0.05) in comparison with the respective controls. Re-produced with permission from [132]. n nThe correct text for the legend of Figure 3 on page 22 is: n nFigure 3. D-Xylose and Compounds 19, 21 and 24 activate AS160. Whole cell content of AS160 and pThr642-AS160 was determined by Western blot analysis in samples that were prepared from L6 myo-tubes, as described in the legend to Figure 1. Representative blot and a summary of n = 3 (* p < 0.05) in comparison the respective controls. Reproduced with permission from [132].